The Many Faces of Defeasibility in Defeasible Deontic Logic

Deontic logic is the logic of obligations, i.e. reasoning about what should be the case. Defeasible logic is the logic of default assumptions, i.e. reasoning about what normally is the case. In defeasible deontic logic these two are combined. An example of this combination is the sentence ‘normally, you should do p’. Now the problem is what to conclude about somebody who does not do p? Is this an exception to the normality claim, or is it a violation of the obligation to do p? This confusion arises because there is a substantial overlap between deontic and defeasibility aspects. In this article we analyze this overlap, and we also show that this confusion can be avoided if one makes the proper distinctions between different types of defeasibility. Furthermore, we also show that these distinctions are essential for an adequate analysis of notorious contrary-to-duty paradoxes such as the Chisholm and Forrester paradoxes.

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